1. Field of the Invention
The present invention relates to procedures for testing structural materials used in the construction industry, and particularly to a method of ascertaining fully grown passive film formation on steel rebar embedded in concrete using electrochemical impedance spectroscopy (EIS).
2. Description of the Related Art
Electrochemical impedance spectroscopy (EIS) (sometimes also referred to as “dielectric spectroscopy” or “impedance spectroscopy”) is a technique for measuring the dielectric properties of a medium as a function of frequency. EIS is based on the interaction of an external electric field with the electric dipole moment of a sample, typically expressed by electrical permittivity. EIS is often used as an experimental method for characterizing electrochemical systems. The technique measures the impedance of a system over a range of frequencies, and therefore the frequency response of the system (including the energy storage and dissipation properties) is revealed. Typically, data obtained by EIS is expressed graphically in a Bode plot or a Nyquist plot.
FIG. 2 illustrates a conventional three-electrode electrochemical cell for electrochemical impedance measurement by EIS. Cell 100 contains an electrolyte solution 108 in which a working electrode 102 and a counter electrode 104 are immersed. Typically, the working electrode 102 and the counter electrode 104 are parallel plate electrodes. In addition to the working electrode 102 and the counter electrode 104, a third voltage reference electrode 106 is placed close to the polarization layer (i.e., the region of positive polarization 110 near the working electrode 102) and measures the voltage difference of the polarization double layer capacity to the working electrode 102. The working electrode 102 is made of the metal to be characterized in combination with the electrolyte 108. The reference electrode 106 may be, for example, an open-tipped glass capillary filled with a standard electrolyte coupled to a standard metal in order to create a defined electrochemical potential to the electrolyte.
The total potential drop across the cell is summed up by all contributions of the chemical process, including mass transport, chemical and adsorption steps, electron transfer, etc. By measuring the impedance spectrum:
            V      REF      *        ⁡          (      ω      )                  I      s      *        ⁡          (      ω      )      over angular frequency range ω and fitting it with an equivalent circuit model, the several process contributions can be separated from each other. The typical evaluation includes determination of Warburg impedance related to mass transport, electron transfer resistance, electrolyte resistance and double layer capacity. As the electrochemical reaction takes place on the working electrode 102, it is necessary to keep the DC potential VREF at a defined value, or alternatively, apply a constant DC current to the cell. This is often performed with a potentiostat/galvanostat DC circuit.
Electrochemical impedance spectroscopy has been used to characterize the nanoscale passive film formation on steel rebar in concrete at different stages of exposure in simulated concrete pore solution (SPS). However, since the rebar is placed in SPS, rather than an actual concrete environment, the EIS results are typically not representative of the actual behavior. It would be desirable to be able to characterize nanoscale passive film formation on steel rebar in its actual concrete environment.
Thus, a method of ascertaining fully grown passive film formation on steel rebar embedded in concrete solving the aforementioned problems is desired.